The NHTSA has upped its investigation of two Ford minivan models.

The National Highway Traffic Safety Administration is stepping up its investigation of more than 82,000 Ford Freestar and Mercury Monterey minivans from the 2004 model year for a sever corrosion issue. The NHTSA announced on Friday that it is upgrading its probe of the vans from an "investigation" to an "engineering analysis." The NHTSA began its probe of the 2004 Freestar and Monterey in December 2011. Ford's minivan twins came under scrutiny after the NHTSA received several complaints that the vans' rear wheel wells were subject to severe corrosion. The agency has now collected 22 allegations of severe rust from the salt-belt states, with some complaints alleging a complete separation of the third-row seat anchor plate from the body of the vehicle. The NHTSA is also taking a look at Ford vans from the 2005-07 model years as they share a similar seat design, according to The Detroit News. No crashes or injuries have been linked to the possible design flaw.